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CADFEM ANSYS extension for computation of hydrodynamic bearings

 

Mastering complexity

The structural behaviour of shafts with journal bearings depends to a high degree on the dynamic characteristics of the lubricant film in the bearings. An established method for the simulation of the lubricant film formation is elastohydrodynamics.

With Tribo-X inside ANSYS journal bearings can be analysed quickly and easily within ANSYS Mechanical. The goal is the correct dimensioning of the bearings as well as the realistic consideration of speed dependent stiffness and dampening effects within dynamic FE analysis.

Main benefits

  • Complete integration of Tribo-X inside ANSYS for the analysis of journal bearings
  • Short run times due to the high performance of the Tribo-X solver
  • Consideration of the elastic deformation of the bearing during operation
  • Identification of rotational speed dependent bearing coefficients
  • Parametric workflow

 

Technical Information

System requirement:
ANSYS Workbench Version 18

Coupling of hydrodynamics and FEA

The analysis system ‘Hydrodynamic Bearing’ couples both physical domains, hydrodynamics in the lubrication gap and deformation of the structural surrounding of the lubrication gap by the help of finite element analysis. Within this process the generalized Reynolds differential equation is solved by the Tribo-X solver. All bearing characteristics are described in ANSYS Workbench using the geometry model and technological factors such as loading and rotational speed. They are implemented in the Workbench project tree as parameters.

Journal bearing coefficients

The vibration behaviour of complex shafts mounted with journal bearings is mainly influenced by the stiffness and damping characteristics of the bearing lubricant. The related coefficients are directly linked to the hydrodynamic load capacity of the lubricant which results from a given rotational speed. Depending on the loading situation and rotational speed of the shaft the coefficients may vary significantly which results in different system behaviours. That is why Tribo-X inside ANSYS computes for each operating condition the representative stiffness and damping terms of the lubricant which will be transferred to the FE model in ANSYS.

The results of the Tribo-X analysis enable an assessment of the load capacity of the journal bearing. Fast variation analyses help to adjust the bearing design according to the loading scenario. For the identified bearing design the  coefficients can be directly transferred to ANSYS Mechanical considering rotational speed dependent stiffness and damping coefficients for further rotordynamic analyses. The consideration of elastic deformation of the bearing allows a more precise computation of the lubricant gap which in turn leads to a more reliable bearing design.

 

Bearing deformation (EHD)

Since bearing deformation due to lightweight design or high loading conditions may influence the lubricant film formation, it is important to take them into account. Tribo-X inside ANSYS allows to include such influences and thus, the execution of an elastohydrodynamic (EHD) computation. Elastic deformation especially due to low rotational speed of the shaft, high loading and low structural stiffness can be represented. By doing this, the identification of an optimal bearing design as well as prevention of bearing damages are promoted.

Cavitation and load capacity

In divergent lubrication gaps cavitation can occur. For a correct computation of the heat balance or bearing coefficients the cavitation effects should be represented by adequate algorithms. These are implemented in Tribo-X which allows a combination of a high prediction quality of the results and ultra-short computing times.

Parameter studies

The high performance capabilities of the EHD solver Tribo-X lead to ultra-short computation times. That is why parametric sensitivity studies can be done easily inside ANSYS. In a first step, a variation analysis may focus firstly on the bearing itself in order to narrow down the bearing choice. In the second step, a continuous workflow coupling the bearing computation as well as the FE analysis, enables to consider design parameters of the bearing and its shaft simultaneously. This allows a holistic optimal design of bearing, shaft and housing.

 

Strong technological partnership

For more than 50 years the Chair of Machine Elements and Tribology of the Institute for Machine Design has been actively working to address technical and scientific issues related to friction, wear and lubrication of different machine elements. Tribo-X is a software package developed by this chair for 3D-simulation of TEHD-contacts.

www.tribo-x.com

 

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